Project Summary Aggregation and propagation of misfolded proteins in the form of abnormal inclusions is a common feature of numerous neurodegenerative diseases. A majority of brains with frontotemporal lobar degeneration (FTLD) are characterized by mislocalization and aggregation of transactivation response element DNA binding protein-43 (TDP-43) into insoluble inclusions. Given that TDP-43 is among the most recently identified pathologic precipitates, the goal of Cycle 1 of this project was to explore the relationship between TDP-43 inclusions, cellular alterations, cortical atrophy and disease phenotype in FTLD and in a conditionally transgenic mouse model expressing wild-type human (h) TDP-43 in the forebrain. We observed that density of TDP-43 inclusions, neuronal shrinkage, and density of activated microglia in cortical gray and white matter are concordant with patterns of cortical atrophy and disease phenotype and that TDP-43 inclusions may spread trans-synaptically. In TDP-43 transgenic mice, we confirmed regionally selective cortical atrophy, and progressive TDP-43 accumulation and inclusion formation, gliosis, apoptosis and behavioral alterations consistent with the pathologic and behavioral alterations in FTLD. Importantly, the density of activated microglia displayed the closest and most consistent relationship with cortical atrophy and disease phenotype in FTLD. Cycle 2 of this project builds upon the observations during Cycle 1 and will focus on the role of microglia / immune activation, including synaptic pruning as disease spreads. We will conduct a comprehensive survey of microglia / immune gene and protein expression changes in brains of FTLD and hTDP transgenic mice, will explore alterations in synapses and synaptic proteins in FTLD and potential role of microglia mediated pruning in synaptic loss, and will investigate the role of microglia generated exosomes in spread of TDP-43 pathology in the mouse model. We propose to base this investigation on rigorous quantitative methods, including unbiased stereological quantification and RNA sequencing (RNAseq) in a unique set of autopsy specimens with extensive clinical and pathological information and in a mouse model which recapitulates features of human disease. The specific aims of the proposed research will test the following hypotheses: Aim 1. FTLD- TDP and hTDP-43 transgenic mouse brains will display regionally selective alterations in microglia transcriptome and associated immune proteins. Aim 2. FTLD-TDP and hTDP-43 transgenic mice will display substantial, early and regionally specific loss of synapses that is due in part to abnormal synaptic pruning by microglia. Aim 3. hTDP-43 transgenic mice will display trans-synaptic spread of TDP-43 pathology over time in the hippocampus and microglia derived exosomes facilitate this spread in the hippocampus and across cortex. The proposed research will generate a great deal of information on the status of synapses and potential trans- synaptic spread of TDP pathology as well as the role of microglia in these processes and in immune alterations. The findings have the potential to illuminate pathways that can serve as therapeutic targets.